Abstract

Abstract

The overall goal is to design a compact chamber to deposit high quality aluminum coatings on 6.5 -- 8.4 m mirrors. The prospects for coating the mirrors in-situ in the telescope were discussed by Davison and Williams in Technical Memo UA-87-21, and by Hill and Lesser in Technical Memo UA-88-06, and by Davison in Technical Memo UA-88-15. A series of experiments to validate the design concepts have been described by Atwood and Sabol (1992) and by Sabol et. al in Technical Memo UA9305. This memo summarizes the various issues related to scaling and our strategy for producing high quality coatings. The LBT/MMT technical efforts since 1988 have concentrated not on how to aluminize with the mirror in the telescope, but on how to aluminize in a compact vacuum chamber (compact length compared to the diameter of the mirror) and on how to evaporate even higher quality aluminum coatings. Unfortunately, no single document assembles the strategy and all the techical details in one place.

Our main strategies are: to produce a high quality, oil-free vacuum in front of the mirror; to separate the relatively dirty vacuum in the cell behind the mirror; to evaporate aluminum at a high deposition rate; to use an array of high-capacity filaments close to the substrate; to bring the vacuum bell jar to the mirror and its cell in the telescope; to deposit the aluminum with the mirror horizon-pointing; and to use cryopanels to achieve high pumping rates at low cost.

Priorities

1. Safety of personnel, optics and equipment.
2. Coatings of the highest quality.
3. Operational efficiency.
4. Affordability
5. Project completion in a timely manner.

Performance Goals

1. high reflectivity coatings
2. low emissivity coatings
3. uniform thickness coatings
4. regular cleaning
5. careful maintenance
6. protection from the weather
7. regular recoating
8. safe operation

Aluminizing Steps

1. Prepare telescope and vacuum chamber.
2. Strip old coating from the mirror.
3. Install mirror in vacuum chamber.
4. Pump down to 1 milliTorr.
5. Glow discharge clean the glass surface.
6. Pump rapidly down to 1 microTorr.
7. Evaporate the aluminum coating onto the mirror.
8. Release the vacuum.
9. Reassemble the telescope.

Possible Scaling Problems

1. Larger diameter vacuum vessel to accomodate larger mirrors.
2. Longer vacuum vessel to accomodate uniform deposition rings.
3. Coating quality degradation from increased mean free path of aluminum.
4. Coating quality degradation from adsorption during slower deposition.
5. Larger filament loads may create wetting/dripping problems.
6. Substrate damage may result from filament breakage/drips.
7. Increased outgassing from the honeycomb mirror and its supports.
8. Oil contamination from the pumping system.
9. Power requirements increase due to area and deposition rate.
10. Uniformity of coating requirements are tightened to 5 nm rms.
11. Uncertainty in scaling glow discharge designs.
12. Manual operations may now take days rather than hours.
13. Stripping the old coating involves larger quantities of nasty chemicals.
14. Hydrogen production during high rate evaporation.
15. Low voltage -- high current power distribution.

Strategies for the Aluminizing System

1. Increase the vacuum quality to 1 microTorr with a two section chamber.
2. Use oil-free high vacuum pumps (cryosorption) to minimize contamination.
3. Increase the deposition rate to >40 Angstroms/second.
4. Use an array of filaments close to the substrate (near-field).
5. Coat the mirror horizon-pointing to minimize problems with drips.
6. Move the bell jar rather than the honeycomb mirror and its cell.
7. Carefully monitor the pumping, cleaning and deposition processes (partial pressure of oxygen, etc.).
8. Use weekly CO₂ snow cleaning to preserve the coating.
9. Use high capacity filaments to reduce handling during reloading.
10. Evaporate aluminum immediately after plasma cleaning.
11. Build a shuttered filament array (or equivalent) for preheating.
12. Retain chamber options for silver, chrominum, gold, etc.

New Technology to Observatory Experience

• Handling 6.5 -- 8.4 meter honeycomb mirrors. This is the only technology that is actually new.

• [Large Bell Jars 7 -- 9 meter bell jars and/or vacuum chambers are new to the observatory. This size of chamber has been around for a number of years for space simulation. To avoid contamination of the satellites, these chambers often must exceed the quality of vacuum needed for aluminization.

• Cryosorption pumping is relatively standard technology. The goal of our design is to build an economical system for operation once or twice per year.

• Glow discharge cleaning of a large area may have scaling problems. We are investigating other options such as r.f. generated plasma cleaning, or not using glow discharge.

• Near-field filament arrays have not previously been used in coating large mirrors. We are comparing the results of computer modelling with actual chamber results.

• Noble gases may relax the pumpdown requirements after glow discharge. Aluminizing under argon bombardment may also increase coating density.

• High-capacity filaments may allow multiple aluminization shots without labor-intensive reloading. These designs may also reduce filament-filament variations.

• Filament transformers to allow high-voltage low-current power distribution with reduced sensitivity to resistance variations.

Baseline Aluminizing System Features

• in-situ coating of primary mirror
• tranportable bell jar
• mirror cell forms part of vacuum vessel
• 250 -- 300 elements, 400 -- 600 Watts each
• 4 -- 5 rings of elements
• 1.0 -- 1.5 meters above the mirror
• filament shutters and baffles
• strip mirror horizon pointing
• install chamber horizon pointing
• clean mirror horizon pointing
• aluminize horizon pointing
• chamber divided into clean / dirty sections
• blowout valve between sections
• roughing pumps: Roots blowers backed by rotary pumps
• hivac pumps: cryopumps (MMT) or cryopanels (LBT)
• filament transformers
• high capacity filaments
• residual gas analyzer
• quartz crystal thickness monitors
• pressure gauges
• bleed valves
• edge seals for ventilation, washing, aluminizing
• dual O-ring flange seal
• design telescope for easy chamber access

Publications

• B. A. Sabol, B. Atwood, J. M. Hill, J. T. Williams, M. P. Lesser, P. L. Byard and W. B. Davison, ``Evaporative Coating Systems for Very Large Astronomical Mirrors'', Proc. S.P.I.E., 1236, pp. 940-951, 1990.

• B. Atwood and B. A. Sabol, ``Studies of Some Aspects of Aluminizing Large Astronomical Mirrors'', Proceedings of the ESO Conference on Progress in Telescope and Instrumentation Technologies, ed. M.-H. Ulrich, pp. 265-268, (Garching:ESO), 1992.

Technical Memoranda

• W. Davison and J. T. Williams, UA-87-21, ``Basic Considerations for the Design of a Vacuum Aluminizing Chamber for the Columbus Project'', Columbus Project Technical Memo.

• J. M. Hill and M. P. Lesser, UA-88-06, ``Notes on Aluminizing 8-meter Mirrors'', Columbus Project Technical Memo.

• W. Davison, UA-88-15, ``Definition of the Aluminizing Chamber for Cost Estimate'', Columbus Project Technical Memo.

• B. A. Sabol, MMT 88-3, ``Mirror Coating at the Sunnyside Facility ....'', MMT Internal Technical Memorandum, September 1988.

• B. A. Sabol, #88-4, ``Status of Research on the Aluminization of Large Astronomical Mirrors: Source Geometry Design Code and Coating Profile Measurements'', MMT Upgrade/Conversion Technical Memorandum, October 1988.

• B. A. Sabol, {\bf Technical Report 24}, ``A Reference Object Technique for Emissivity Measurements with the Thermographic Camera'', MMTO Technical Report, September 1989.
• B. A. Sabol, #89-1, ``Glow Discharge Cleaning - Review and Recommendations'', MMT Upgrade/Conversion Technical Memorandum, December 1989.

• P. Byard and B. Atwood, OSU-89-01, ``Aluminizing Large Optics'', Columbus Project Technical Memo.

• E. Melsheimer and M. Clark, No. 15, ``Mirror Coating Facilities'', Magellan Project Memo, January 1990.

• B. Atwood and B. Sabol, OSU-91-01, ``Cleaning and Coating Large Mirrors'', Columbus Project Technical Memo.

• W. Gallieni, ``Washing and Aluminizing Equipment Storage'', ADS Report to Columbus Project, March 1991.

• B. A. Sabol, J. M. Hill, B. Atwood, P. L. Byard, T. P. O'Brien, J. A. Mason, UA-93-05, ``Aluminization Research for the Large Binocular Telescope'', Large Binocular Telescope Project Technical Memo.